This research is aimed at developing a three component model of heavy metal adsorption and desorption to soils and suspended particles. No comprehensive method exists to predict from soil or particle properties the extend of adsorption, and more importantly, the extent and quantity of total desorbable metal. no empirical rules exist for estimating either adsorption or desorption partition coefficients. Yet these parameters are of critical importance in establishing the rate at which chemicals migrate from contaminated Superfund sits and the resulting concentration in the ground water. Hence the extent of exposure to humans via ground water is directly tied to the magnitude of these parameters. In surface water the fraction of total chemical that is dissolved is similarly determined by the partition coefficient. Since only dissolved chemical is directly bioavailable this parameter controls the exposure concentration of the biota. Hence it directly influences the extent of bioaccumulation of chemicals. The development of the model will be based on three site description of sorption. The metals being considered, nickel, cadmium, mercury and chromium, are the same as in the other components of this research project. The Langmuir binding strength and capacity constant parameters will be related to the quantity of extractable amorphous iron and manganese oxides and organic carbon in the soil. The desorbable fraction of the metals will be determined by a hydroxylamine extraction which has been successful in quantifying the bioavailability and sediment - interstitial water partitioning of metals in lakes and estuarine sediments. It is anticipated that the results of this component will provide the sorption isotherm parameters and desorbable fractions to the modeling component for use in the fate and transport models.